Hey there, if you’ve ever used a screw jack to lift something heavy—like a truck for repairs, a stage for a concert, or even a construction formwork at a job site—you might’ve wondered: how do I actually figure out how much torque I need to turn that handle (or motor) to make it work? Spoiler: it’s not just guessing how strong your arm is, and if you get it wrong, you could either not lift the load at all or burn out your jack before you even get started. As a screw jack supplier, I get questions about this all the time—people think it’s rocket science, but it’s actually pretty straightforward once you break it down. Let’s walk through it like we’re chatting over a coffee (no fancy equations that’ll make your eyes glaze over, promise). Screw Jack
First, let’s get one thing straight: screw jacks aren’t magic. They use that classic simple machine trick—screws as inclined planes—to turn rotational force (that’s torque, by the way) into linear force (the actual lifting power). So torque is what you put in, and the lifting force is what you get out. But to calculate how much torque you need, you’ve got to know a few key numbers first, and no, you don’t need a PhD in engineering. Let’s list ‘em out like we’re checking items off a to-do list: the load you’re lifting, the screw’s lead, the screw’s pitch diameter, and the coefficient of friction between the screw and the nut. That’s it—four main things.
Wait, let’s define those quick, ‘cause “lead” and “pitch” are terms everyone mixes up. Pitch is the distance between two adjacent threads on the screw, right? Lead is how far the nut moves in one full turn of the screw. For a single-start screw (the most common one you’ll find on standard screw jacks), lead equals pitch. If it’s a multi-start screw (like for faster lifts), lead is more than pitch—we don’t need to overcomplicate that for this basic calculation, just remember single-start = lead = pitch for most everyday jobs. Got it? Good.
Now, let’s start with the ideal case—no friction, just pure mechanical advantage. That’s the easiest way to wrap your head around it. The ideal torque (let’s call it T_ideal) is what you’d need if there was zero friction, which never actually happens, but it’s a starting point. The formula for that is T_ideal = (Load × Lead) / (2 × π). Let’s plug in a real example to make this concrete. Say you’ve got a 5,000 lb load (that’s about the weight of a small pickup, for reference) on a single-start screw jack with a lead of 0.2 inches. Let’s do the math: (5000 × 0.2) / (2 × 3.14) = 1000 / 6.28 ≈ 159.2 lb-in. That’s the torque if there was no friction. But here’s the catch: friction is always there—between the screw and nut, between the nut and the jack’s housing, even a tiny bit from the load pushing down on the assembly. That ideal number is way too low for real life, so we have to add friction into the mix.
Now, let’s talk about friction. The coefficient of friction (let’s call that μ) is just a number that tells you how slippery two surfaces are. For steel screws running against a standard bronze nut (that’s the most common pairing in screw jacks, trust me), μ is usually between 0.1 and 0.15. If you’re using a different nut material—like plastic for light loads—it’s lower, but bronze is the go-to for durability and consistency. We also need the pitch diameter (let’s call that D) of the screw, which is basically the average diameter of the threads—for a standard 1-inch diameter screw, D is usually around 0.875 inches, but you can find this in your screw jack’s specs (we always post that for every model we sell, by the way, so you don’t have to guess).
The formula for actual torque (T_actual, which is what you’ll really need) adjusts that ideal number to account for friction. The full formula is T_actual = (Load × (Lead/(2×π) + μ × (D/2))) / (1 – (μ × Lead)/(π × D)). Wait, I know that looks scary, but let’s plug in our same example to make it simple. Let’s say our screw has a pitch diameter of 0.875 inches, μ = 0.12 (right in the middle of that bronze range). Let’s calculate step by step:
First, the top part of the numerator: Lead/(2×π) is still ~0.0318 inches. μ × (D/2) is 0.12 × (0.875/2) = 0.12 × 0.4375 = 0.0525 inches. Add those together: 0.0318 + 0.0525 = 0.0843 inches. Multiply by load (5000 lb): 5000 × 0.0843 = 421.5 lb-in. That’s the top.
Now the denominator: 1 – (μ × Lead)/(π × D). Plugging in numbers: (0.12 × 0.2) / (3.14 × 0.875) = 0.024 / 2.7475 ≈ 0.00873. So 1 – 0.00873 ≈ 0.99127.
Now divide the top by the bottom: 421.5 / 0.99127 ≈ 425 lb-in. Oh, that’s way higher than the 159 lb-in ideal! That’s why friction is such a big deal—you can’t ignore it. And notice I used a middle-of-the-road μ; if your surfaces are rusty or dirty, μ could jump to 0.2 or more, so your torque would be even higher. That’s why it’s smart to use a lubricant too—grease or anti-seize can cut that friction down, so you need less torque, which is easier on your jack and the drive system.
Wait, but what about when you’re lowering the load? That’s a different scenario, right? When you lower, the friction works against you instead of with you, so you need less torque (or even none, if the screw is self-locking). Oh right, self-locking is a key point for screw jacks—if the lead of the screw is small enough compared to the friction, the load won’t unwind the screw on its own, which is what you want for safety. If your screw isn’t self-locking, you’ll need to apply torque to hold the load as you lower it, so you can’t just use the lowering torque number and call it a day. As a supplier, I always check for self-locking when we recommend jacks—you don’t want a load crashing down ‘cause you picked the wrong screw type.
Let’s also talk about real-world factors that the formula doesn’t account for, ‘cause formulas are great, but actual use matters. For example, if you’re lifting a load that’s not perfectly centered on the jack, that creates side load on the screw, which increases friction. If you’re using a gearbox to drive the jack (instead of a manual handle), you have to add in the efficiency of the gearbox too—gearboxes aren’t 100% efficient, so some torque is lost there. We always include efficiency ratings for our gear-driven screw jacks on our product pages, so you can adjust your calculations for that. Another thing: wear and tear. A jack that’s been used for years has worn threads, which increases friction, so you’ll need more torque over time. That’s why regular lubrication and inspection are important—we send a quick care sheet with every jack we sell, just to keep things running smoothly.
Maybe you’re thinking, “This is cool, but I don’t want to do all this math!” Fair enough. That’s why all the screw jack models we supply have a torque rating listed right in the specs. Let’s say you have a 5,000 lb load—if you check our 1-ton screw jack (wait, 1 ton is 2,000 lbs, so make that a 3-ton jack) specs, you’ll see it needs about 400-450 lb-in of torque to lift that load, which lines up with the example we did earlier. That’s the beauty of working with a supplier who’s actually got real-world numbers, not just textbook formulas—we’ve tested these jacks with actual loads, so you don’t have to crunch the numbers yourself.
Wait, let’s go over a common mistake people make: only using the ideal torque and forgetting friction. A customer once told me they picked a jack based on the ideal torque, and it wouldn’t even move a 1,000 lb load. Turns out they forgot about friction, so the actual torque needed was like three times what they thought. That’s why we always tell customers to add a 20-30% safety factor to their calculated torque. Stuff happens—threads aren’t perfectly smooth, lubricant wears off, the load shifts a little. Adding that safety margin means you don’t get stuck mid-lift, or overload your motor or handle.
Let’s wrap this up with a quick recap, ‘cause I know that formula felt like a lot. Here’s the step-by-step cheat sheet we give to every customer: 1. Get your actual load weight (don’t guess—weigh it if you can, it’s worth it). 2. Get the lead and pitch diameter of your screw (if you’re buying a jack from us, we send that to you). 3. Use the standard bronze nut friction number (0.1-0.15) for most jobs. 4. Plug into the actual torque formula (or just use our pre-tested specs, which is way easier). 5. Add 25% safety factor minimum. 6. If lowering, remember self-locking and adjust accordingly.
If all this math still makes your head spin, that’s exactly why we’re here. We don’t just sell screw jacks—we help you pick the right one for your load, and walk you through any torque questions you have. Whether you need a manual jack for occasional use, or a motorized one for heavy industrial jobs, we’ve got you covered. No confusing jargon, no hidden fees, just straight answers to make sure your lift goes off without a hitch. If you’re working on a project and need help figuring out the right torque or jack size, just reach out and chat with our team—we’re happy to help. We’ve been in this game long enough to know that getting the torque right isn’t just about making the jack work, it’s about keeping you safe and your project on track. Don’t overcomplicate it, but don’t cut corners either— that’s the sweet spot.
Stepper Motor References: 1. Shigley, J. E., & Mischke, C. R. (2001). Mechanical Engineering Design (6th ed.). McGraw-Hill. 2. American Society of Mechanical Engineers. (2017). ASME B106.1M-2017: Design of Screw Jacks. ASME. 3. Halliday, D., Resnick, R., & Walker, J. (2021). Fundamentals of Physics (12th ed.). Wiley.
Hangzhou ANG Drive Co., Ltd.
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Address: No.185, Jincheng Rd., Hangzhou 311202, China
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WebSite: https://www.anjodrive.com/
